Add minimum cache TTL for successful DNS responses

CHANGELOG.next.asciidoc

@@ -341,6 +341,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Add the `overwrite_keys` configuration option to the dissect processor. {pull}19464[19464]
 - Add support to trim captured values in the dissect processor. {pull}19464[19464]
 - Added the `max_cached_sessions` option to the script processor. {pull}19562[19562]
+- Add minimum cache TTL for successful DNS responses. {pull}18986[18986]
 
 *Auditbeat*
 

libbeat/processors/dns/cache.go

@@ -35,8 +35,9 @@ func (r ptrRecord) IsExpired(now time.Time) bool {
 
 type ptrCache struct {
 	sync.RWMutex
-	data    map[string]ptrRecord
-	maxSize int
+	data          map[string]ptrRecord
+	maxSize       int
+	minSuccessTTL time.Duration
 }
 
 func (c *ptrCache) set(now time.Time, key string, ptr *PTR) {
@@ -135,11 +136,10 @@ func (ce *cachedError) Cause() error  { return ce.err }
 // reverse DNS queries. It caches the results of queries regardless of their
 // outcome (success or failure).
 type PTRLookupCache struct {
-	success    *ptrCache
-	failure    *failureCache
-	failureTTL time.Duration
-	resolver   PTRResolver
-	stats      cacheStats
+	success  *ptrCache
+	failure  *failureCache
+	resolver PTRResolver
+	stats    cacheStats
 }
 
 type cacheStats struct {
@@ -155,8 +155,9 @@ func NewPTRLookupCache(reg *monitoring.Registry, conf CacheConfig, resolver PTRR
 
 	c := &PTRLookupCache{
 		success: &ptrCache{
-			data:    make(map[string]ptrRecord, conf.SuccessCache.InitialCapacity),
-			maxSize: conf.SuccessCache.MaxCapacity,
+			data:          make(map[string]ptrRecord, conf.SuccessCache.InitialCapacity),
+			maxSize:       conf.SuccessCache.MaxCapacity,
+			minSuccessTTL: conf.SuccessCache.MinTTL,
 		},
 		failure: &failureCache{
 			data:       make(map[string]failureRecord, conf.FailureCache.InitialCapacity),
@@ -198,11 +199,14 @@ func (c PTRLookupCache) LookupPTR(ip string) (*PTR, error) {
 		return nil, err
 	}
 
+	// We set the ptr.TTL to the minimum TTL in case it is less than that.
+	ptr.TTL = max(ptr.TTL, uint32(c.success.minSuccessTTL/time.Second))
+
 	c.success.set(now, ip, ptr)
 	return ptr, nil
 }
 
-func max(a, b int) int {
+func max(a, b uint32) uint32 {
 	if a >= b {
 		return a
 	}

libbeat/processors/dns/cache_test.go

@@ -19,8 +19,8 @@ package dns
 
 import (
 	"io"
-	"strings"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 
@@ -30,12 +30,14 @@ import (
 type stubResolver struct{}
 
 func (r *stubResolver) LookupPTR(ip string) (*PTR, error) {
-	if ip == gatewayIP {
+	switch ip {
+	case gatewayIP:
 		return &PTR{Host: gatewayName, TTL: gatewayTTL}, nil
-	} else if strings.HasSuffix(ip, "11") {
+	case gatewayIP + "1":
 		return nil, io.ErrUnexpectedEOF
+	case gatewayIP + "2":
+		return &PTR{Host: gatewayName, TTL: 0}, nil
 	}
-
 	return nil, &dnsError{"fake lookup returned NXDOMAIN"}
 }
 
@@ -98,4 +100,29 @@ func TestCache(t *testing.T) {
 		assert.EqualValues(t, 3, c.stats.Hit.Get())
 		assert.EqualValues(t, 3, c.stats.Miss.Get()) // Cache miss.
 	}
+
+	minTTL := defaultConfig.CacheConfig.SuccessCache.MinTTL
+	// Initial success returned TTL=0 with MinTTL.
+	ptr, err = c.LookupPTR(gatewayIP + "2")
+	if assert.NoError(t, err) {
+		assert.EqualValues(t, gatewayName, ptr.Host)
+
+		assert.EqualValues(t, minTTL/time.Second, ptr.TTL)
+		assert.EqualValues(t, 3, c.stats.Hit.Get())
+		assert.EqualValues(t, 4, c.stats.Miss.Get())
+
+		expectedExpire := time.Now().Add(minTTL).Unix()
+		gotExpire := c.success.data[gatewayIP+"2"].expires.Unix()
+		assert.InDelta(t, expectedExpire, gotExpire, 1)
+	}
+
+	// Cached success from a previous TTL=0 response.
+	ptr, err = c.LookupPTR(gatewayIP + "2")
+	if assert.NoError(t, err) {
+		assert.EqualValues(t, gatewayName, ptr.Host)
+		// TTL counts down while in cache.
+		assert.InDelta(t, minTTL/time.Second, ptr.TTL, 1)
+		assert.EqualValues(t, 4, c.stats.Hit.Get())
+		assert.EqualValues(t, 4, c.stats.Miss.Get())
+	}
 }

libbeat/processors/dns/config.go

@@ -87,6 +87,9 @@ type CacheSettings struct {
 	// from the DNS record.
 	TTL time.Duration `config:"ttl"`
 
+	// Minimum TTL value for successful DNS responses.
+	MinTTL time.Duration `config:"min_ttl" validate:"min=1"`
+
 	// Initial capacity. How much space is allocated at initialization.
 	InitialCapacity int `config:"capacity.initial" validate:"min=0"`
 
@@ -122,6 +125,9 @@ func (c *Config) Validate() error {
 
 // Validate validates the data contained in the CacheConfig.
 func (c *CacheConfig) Validate() error {
+	if c.SuccessCache.MinTTL <= 0 {
+		return errors.Errorf("success_cache.ttl.min must be > 0")
+	}
 	if c.FailureCache.TTL <= 0 {
 		return errors.Errorf("failure_cache.ttl must be > 0")
 	}
@@ -146,6 +152,7 @@ func (c *CacheConfig) Validate() error {
 var defaultConfig = Config{
 	CacheConfig: CacheConfig{
 		SuccessCache: CacheSettings{
+			MinTTL:          time.Minute,
 			InitialCapacity: 1000,
 			MaxCapacity:     10000,
 		},

libbeat/processors/dns/docs/dns.asciidoc

@@ -51,6 +51,7 @@ processors:
     success_cache:
       capacity.initial: 1000
       capacity.max: 10000
+      min_ttl: 1m
     failure_cache:
       capacity.initial: 1000
       capacity.max: 10000
@@ -80,6 +81,9 @@ the memory for this number of items. Default value is `1000`.
 cache can hold. When the maximum capacity is reached a random item is evicted.
 Default value is `10000`.
 
+`success_cache.min_ttl`:: The duration of the minimum alternative cache TTL for successful DNS responses. Ensures that `TTL=0` successful reverse DNS responses can be cached.
+Valid time units are "ns", "us" (or "µs"), "ms", "s", "m", "h". Default value is `1m`.
+
 `failure_cache.capacity.initial`:: The initial number of items that the failure
 cache will be allocated to hold. When initialized the processor will allocate
 the memory for this number of items. Default value is `1000`.